1. Field of the Invention
The invention relates to the control of a transmission in a vehicle powertrain having a step ratio power flow path and an infinitely variable ratio power flow path.
2. Background Art
An example of a vehicle powertrain having fixed ratio gearing and continuously variable torque delivery features is disclosed in U.S. Patent publication US2004/0127321, published Jul. 1, 2004. Another example is disclosed in co-pending U.S. patent application Ser. No. 11/318,656, filed Dec. 27, 2005. The patent application corresponding to this patent application publication and the co-pending '656 patent application are assigned to the assignee of the present invention. The disclosures are both incorporated herein by reference and complement the present disclosure.
A typical step ratio transmission in a vehicle powertrain will allow a limited number of engine speeds for a given output shaft speed. The number of engine speeds that are available depends upon the number of gear ratios, which are fixed by a design choice. In contrast, a transmission of the type disclosed in the aforementioned earlier disclosures will allow a large number of engine speeds for a given output shaft speed due to the infinitely variable ratio feature.
A control strategy for achieving an optimum engine speed for maximum engine efficiency for a conventional step ratio transmission is not adaptable to a control strategy for achieving optimum engine speed for a given output shaft speed for an infinitely variable transmission. A transmission having infinitely variable characteristics as well as step ratio gearing, therefore, requires a more complex control strategy to optimize engine speed for maximum efficiency. Provision of such a control strategy is an objective of the invention.